1. Field of the Invention
This invention relates to a motor drive control system for rotating a motor such as a stepping motor with an excitation phase thereof change over, and particularly to a drive control system for stably controlling a plurality of stepping motors for effecting acceleration control, deceleration control, retaining control, etc. with a low driving electric power when the stepping motors are driven at one time.
2. Related Background Art
As a typical heretofore known drive system for a stepping motor, there is a constant voltage drive system. The constant voltage drive system is simple in circuit construction and inexpensive and therefore has been widely adopted.
FIGS. 16 and 17 of the accompanying drawings show the timing of excitation control signals for a 1-2 phase excitation system and a second phase excitation system heretofore generally adopted as the drive system for a stepping motor. Also, FIG. 18 of the accompanying drawings shows the relation of the velocity control of a stepping motor, and shows areas for performing the accelerating operation of changing the frequencies of the excitation control signals shown in FIGS. 16 and 17 to thereby change the number of revolutions of the stepping motor and provide a predetermined rotating velocity within a predetermined time, the constant velocity operation providing a constant rotating velocity, and the decelerating operation stopping rotation within a predetermined time. These series of accelerating, constant velocity and decelerating operations together constitute the operation sequence of the stepping motor.
Here, FIG. 14 of the accompanying drawings and Table 1 show an example of the timing of 1-2 phase excitation and the excitation time during acceleration, and FIG. 15 of the accompanying drawings and Table 2 likewise show an example of the timing and the excitation time during deceleration. As shown in Table 1 and Table 2, generally during the non-driving (t0 in Table 1, and t10 in Table 2) of the stepping motor, control is effected so as to decrease electric power consumption in such a manner as to cut off the excitation phase of the stepping motor and not to supply an electric current to the stepping motor.
Table 1 below shows the excitation time during the acceleration in the drive control of a stepping motor according to the prior art, and Table 2 below shows the excitation time during the deceleration in the drive control of the stepping motor according to the prior art.
In such a state, the deviation or the like between the static stop position of the stepping motor when excitation is off and the dynamic stop position of the stepping motor when excited is a problem and the accuracy of the stop position is aggravated and therefore, when the stepping motor is to be driven again, the state of the second excitation of the rotation initial phase and rotation final phase of 1-2 phase excitation control is once continued for a predetermined time as indicated by t1 and t9 in FIGS. 14 and 15 in order to increase the positioning accuracy of the rotation starting position and rotation stopping position of the stepping motor during the initial phase excitation before the start of acceleration and the final excitation immediately before the termination of deceleration.
FIGS. 10, 11, 12 and 13 of the accompanying drawings show an example of the relation between the winding electrical current flowing to a winding of the stepping motor when acceleration control and deceleration control are effected by the aforedescribed prior-art system and the excitation control signal thereof. Here, when an attempt is made to effect stepping motor acceleration control (hereinafter referred to as the high speed rotation) by the aforedescribed constant voltage driving system as shown in FIG. 11, wherein the driving frequency of the excitation control signal is gradually increased, as is apparent from FIGS. 10 and 11, the rising of the electrical current of the windings of the stepping motor becomes slow and the substantial electrical current of the windings becomes small without the current value being saturated, and as the result, the driving torque of the stepping motor decreases in a high speed rotation area.
Likewise, when an attempt is made to effect deceleration control (hereinafter referred to as the low speed rotation) as shown in FIG. 13 wherein the driving frequency of the excitation control signal is gradually decreased, as is apparent from FIGS. 12 and 13, the influence of the inductance of the windings of the stepping motor is not so great as during the high speed rotation, and the current value is saturated in an area wherein the driving frequency is low and a great electrical current of the windings flows and the driving torque also becomes great. Accordingly, when an attempt is made to effect the driving of the stepping motor from low speed rotation to high speed rotation, the driving torque of the stepping motor decreases during the high speed rotation and therefore, to maintain the torque during the high speed rotation, for example, the driving voltage of the stepping motor is increased or the resistance value of the windings is lowered or the number of the windings is decreased to make the inductance small.
Also, at this time, in order to decrease the noise caused by the excessively great torque during low speed rotation, a method of providing a transistor switch in the power source supply line of the stepping motor, and chopping this transistor to thereby reduce the electrical current of the entire stepping motor is utilized. The transistor switch provided in the power source line needs to have a low voltage drop in the transistor from a property thereof, and such a transistor is expensive and has suffered from the problem of a high cost.
Also, even if these methods are carried out, ideal uniform driving torque from low speed rotation to high speed rotation and the driving current thereof are not obtained and as the result, a decrease in the torque during the high speed rotation is unavoidable and therefore, in reality, stepping motor control from the low speed to the high speed is effected on the basis of this torque. Accordingly, generally the specification of the stepping motor is determined on the basis of the torque for the actual driving current during the high speed rotation and therefore, unexpected excessively great torque and an excessively great electrical current of the windings are created in the initial excitation phase during low speed rotation, particularly at the start of acceleration, and at the final excitation phase or the like at the terminal of deceleration.
Further, when in such a state, an attempt is made to drive two or more stepping motors at a time, there has been the problem that a very great electrical current of the windings of the stepping motors flows and a great current capacity power source becomes necessary. Also, when the capacity of the power source is small, there has been the problem that when the stepping motor is driven, an overcurrent protection circuit is operated and the power source is cut off and in some cases, the power source does not normally operate.
It is an object of the present invention to solve the above-noted problems and to provide a stepping motor drive control system designed to reduce an excessively great driving current to thereby effect the stable rotative driving of the motor.
It is another object of the present invention to provide a stepping motor drive control system for reducing the excessively great driving current created in the initial retaining excitation period during low speed rotation, particularly the acceleration control, of a stepping motor and the final retaining excitation period or the like during the deceleration of the stepping motor to thereby enable a power source of small power capacity to be used as a power source for driving the stepping motor.
Further objects of the present invention will become apparent from the following description of some specific embodiments of the invention.